This invention relates generally to switching amplifiers and, in particular, to apparatus and methods for compensating the output stage in amplifiers of this type so as to reduce back-EMF.
Amplifiers with reactive impedance transformation output stages, as shown in U.S. Pat. No. 5,610,553 entitled xe2x80x9cSwitching Amplifier with Impedance Transformation Output Stage,xe2x80x9d provide high efficiency at high voltage or current levels. The teachings of this patent are incorporated herein by reference.
Unlike conventional amplifiers, however, impedance transformation switching amplifiers do not exhibit symmetrical or constant drive capability. As a result, unless compensated, back-EMF from the load can seriously distort the output waveform. As well, inductance in anti-aliasing output filters commonly used with such switching amplifiers can seriously degrade performance.
A need accordingly exists for a compensation method to counteract asymmetrical and variant output impedance in switching amplifiers of this type.
This invention provides circuitry for reducing and, preferably eliminating, load-induced back-EMF in impedance-transformation switching amplifiers. Different embodiments are deployed, depending upon the severity of the error. Broadly a gated switch coupled to the load is driven by a control circuit appropriately timed to shunt back-EMF. In the event that the amplifier is already equipped with gated switches used to transfer energy representative of an input signal to and from the load impedance, at least some of the same gated switches may conveniently be used for back-EMF control. In preferred configurations, separate gated switches are used on both sides of the load, and the control circuit is configured to drive all of the switches.
If the error is relatively minor, the control circuit may include a timer operative to activate the gated switch after a fixed time interval. Alternatively, the time interval may be varied in relation to the time associated with energy dissipation through a purely resistive load. If the energy storage time of the load is regulated using feedback, the initiation of the shunting action is also preferably feedback-regulated.
If the error is more severe, an error detection circuit is used to determine if the amplifier output voltage exceeds a level representative of a desired input, and circuitry is added for inverting the polarity of the output if the level is exceeded. In accordance with this alternative method, the amplifier output polarity is inverted and its output caused to be a function of the positive error in the circumstance of a substantial positive error, such as that caused by an inductive load.